Various water soluble iodinated radiographic contrast media (RCM) have been studied for their effect on complement components C3 and C4, purified and in serum. Hepatotropic RCM, and at higher concentration also some nephrotropic RCM, were found to exert a direct effect on purified C3 and C4. RCM treated human C3 and C4 are characterized by (a) loss of haemolytic function, (b) retention of activity in the formation of fluid phase C3 convertases and (c) an antigenic relationship to activated C3 and C4 (C3b and C4b, respectively). This direct alteration of C3 and C4 can probably also occur in serum since loss of haemolytic function is observed at similar RCM concentrations after incubation of serum and of purified components. It is concluded that RCM treated C3 and C4 represent altered forms of these components that resemble C4b and C3b in activity and conformation (C3b-like C3 and C4b-like C4). The alteration is probably caused by binding of RCM, exerting a mild denaturing effect. C3b-like C3 is a potential activator of the alternative pathway, and both C3b-like C3 and C4b-like C4 are known to be cleaved by serum inactivators. A possible pathological significance of the generation of C3b-like C3, C4b-like C4 and their split products remains to be evaluated.